Vinylidene chloride interpolymers are well-known in the prior art. Such polymers are also well-known to be thermally sensitive, which means that upon exposure to desirable processing temperatures such polymers tend to thermally decompose, e.g., generate carbonaceous material contamination, discolor or evolve hydrogen chloride.
In the past, the practice has been to extrude polymeric compositions comprising a vinylidene chloride interpolymer directly from the form in which it is recovered. Because of the convenience of shipping and handling, it is desirable to form such polymeric compositions into pellets prior to final extrusion. As the demand for pellets has increased, the demand has increased for a pellet which can withstand the myriad processing conditions to which powdered resins are subjected.
While pellets of polymeric compositions comprising vinylidene chloride interpolymers may be an advantageous form from which to fabricate articles, the pellets of such polymeric compositions are particularly difficult to extrude. Pellet formation requires an exposure of the thermoplastic composition to heat prior to the conventional extrusion step of the polymeric composition into articles. This additional heat history is believed to make the vinylidene chloride interpolymer in pellet form even more susceptible to thermal instability than a vinylidene chloride interpolymer in powder form. Consequently, additive packages which improve the thermal stability of polymeric compositions comprising vinylidene chloride interpolymer in powder form do not necessarily improve the thermal stability of such polymeric compositions in pellet form.
Although satisfactorily extrudable for a relatively short period, it has been found that attempts to extrude vinylidene chloride interpolymer pellets over long periods on certain extrusion equipment have also proven unsatisfactory due to the thermal sensitivity of the vinylidene chloride interpolymer and, consequently, an undesirable level of carbonaceous material contamination, increased discoloration or higher hydrogen chloride in the extrudate.
It is desirable to produce a polymeric composition which is less thermally sensitive than vinylidene chloride interpolymer alone; and consequently, which can be extruded, in either powder or pellet form, into an extrudate which possesses less carbonaceous material contamination, less discoloration or less hydrogen chloride evolvement than an extrudate formed from vinylidene chloride interpolymer alone. It is to this goal that the present invention is directed.